More fully "snow water equivalent". The amount of precipitation represented by a quantity of snow, sleet, or ice.
rare The amount of water, measured in terms of mass, which requires the same amount of heat as a given body in order to raise its temperature by the same amount.
The water equivalent of a particular body is calculated by multiplying the mass of the body by its specific heat.
Mid 19th century (in an earlier sense). From water + equivalent.